For instance a field with options “1” will assign the user who created the tracker item to the field
A field with option “0,1” will let the user to choose a user and to send a mail each time the item is modified (can be convenient for a “assign to field” )
The notification email contains the list of fields with the old and new values for each field that changed.

Display limitation:
tiki >= 1.10
If you want only display some users for a user selector field with no automatic feeding, you can checked the list you want while defining the field.

Note on how to get auto-assignment work in the TRACKER Plugin: In case it did not work as you expected, you could
(1) try to set view=”user” in the plugin
(2) hide the field for everyone except for admin, and add it to the plugin’s fields parameter. This works for both the group and the user selector fields. One current drawback (in Tiki5.0B1): the field labels are shown in both cases. As long as it is not fixed, one could use pretty trackers to circumvent this problem. Find more details in a related bug tracker item.

Select multiple users filterable by groups

Since Tiki 17, the user selector tracker field alows selecting multiple users, and that selection can be on a full list of site users, of filterable by groups of users:

This is also useful for notifications to user creators/owners of a tracker item, to handle permissions per item when no full permissions are granted in the whole tracker level but only to the items that the user is set as creator/owner of the item.